Rubber de-vulcanisation

ABSTRACT

The present invention relates to a method for preferentially breaking cross-links in a vulcanized rubber, thereby de-vulcanizing the rubber, by the use of a supercritical fluid, such as carbon dioxide The supercritical fluid causes swelling of the said vulcanized rubber material, wherein the time of swelling is not more than 50% of the time required for full volume equilibrium swelling of the rubber in said supercritical fluid and the swelling is followed by a rapid reduction of the supercritical fluid pressure The cross links ( 3 ) become fully extended and under strain to hold the internal pressures caused by the solvent swelling affect of the supercritical gas When equilibrium swell has been achieved, the pressure within the processing vessel is rapidly dropped to a predetermined level causing a degassing and expansion of the supercritical fluid that has been absorbed within the vulcanized rubber The resulting three dimensional separation of the rubber molecules will put a further rapid strain on the cross links, causing them to break ( 7 ), thereby giving the affect of de-vulcanization.

The present invention relates to a method for de-vulcanizing avulcanized rubber.

Many millions of tonnes of vulcanised rubber waste are generated worldwide each year from both end of life products such as tyres and fromproduction line waste generated by rubber processors. For many yearsattempts have been made to find ways of recycling such waste into newrubber products for commercial, economic and legislative reasons, butalthough some methods show technical promise, none have been widelyadopted by the rubber industry. The excellent physical and chemicalproperties of vulcanised rubbers that are exploited in a wide range ofproducts make the recycling of these materials extremely difficult.Chemical cross-linking of adjacent rubber molecules results in thermosetproperties and excellent thermal resistance. Vulcanised rubbers cannottherefore be melted and reformed as would be the case with thermoplasticpolymers. In the same way, the stable three-dimensional network ofchemically cross-linked rubber molecules causes vulcanised rubbers to beinsoluble in solvents, permitting only swelling to take place. A furthercharacteristic of vulcanised rubber is that the vulcanisation processutilises the majority of chemically active sites along the rubbermolecules, causing the surface of vulcanised rubber to have low surfaceenergy leading to poor interfacial adhesion to a new rubber matrix.Mixing a ground vulcanised rubber in a virgin rubber matrix thereforegenerally leads to products with lower tear strength than the tearstrength of the virgin rubber. To solve this problem methods have beendeveloped for de-vulcanisation by chemical, biological or mechanicalmeans, the use of heat or high frequency energy, or a combination ofthese methods. In many cases, the techniques described are slow.Alternatively they may employ toxic chemicals or use high levels ofenergy, and tend to be incapable of only cleaving cross-links withoutreducing the molecular weight of the rubber, thus also resulting in asignificant reduction of physical properties.

An object of this invention is to provide a method for treating avulcanised rubber particle such that it can be mixed and cured with avirgin rubber without significantly deteriorating the physicalproperties of the virgin rubber matrix.

According to an aspect of the invention there is provided a method forde-vulcanizing a vulcanized rubber comprising: swelling said material ina supercritical fluid maintained at a supercritical pressure andtemperature, whereby said supercritical fluid causes swelling of thesaid vulcanized rubber material, and a subsequent rapid reduction of thesupercritical fluid pressure characterized in that the time of swellingis not more than 50% of the time required for full volume equilibriumswelling of the rubber in said supercritical fluid.

Accordingly, this invention provides a rapid non-hazardous process fordestroying chemical cross-links in a surface layer of the cured rubberwithout damaging the C—C bonds of the rubber polymer chains.

A further advantage of the invention is that it provides a rapid methodof de-vulcanising the surface layer of cured rubber particles that isnon hazardous to process operators or the environment and uses lowlevels of processing energy.

Without being bound by any theory, it is believed that the presentinvention is based on a rapid, but controlled three dimensionalexpansion of the partly swollen vulcanised rubber, thereby causingpreferential breakage of cross-links at the surface of the rubberparticles.

Preferably the vulcanised rubber is processed in a pressurised vesselcontaining carbon dioxide as a supercritical fluid at an appropriatetemperature and pressure to achieve a solubility parameter suitable toswell the vulcanised rubber to an equilibrium state, as determined bythe cross-link density and the distribution of cross-link types, i.e.cyclic sulphides, monosulphidic, disulphidic or polysulphidic, but mayinstead use other supercritical fluids such as nitrogen or arefrigerant. Once partly swelling conditions have been achieved, thesupercritical fluid undergoes a rapid but controlled decompression,causing gassing of the supercritical fluid and expansion of thevulcanised rubber beyond the level of partly swell determined by theconstraining forces of the cross-links, thereby causing the cross-linksto break. It is reported, by Tobolsky et al. in Polymer Science andMaterials; Wiley-Interscience: New York, 1960 that in cross-linked dienerubber systems, sulphur-sulphur bonds are the weakest chemical bonds.The carbon-carbon single bond energy in peroxide-cross-linked vulcanisedrubber is 93 kcal/mol, the bond energy of a carbon-sulphur-carbon bondin monosulphide cross-linked rubber is 50-60 kcal/mol, whereas the bondenergy of a carbon-sulphur-sulphur-carbon bond in disulphidecross-linked rubber is 35 kcal/mol and the bond energy of acarbon-(sulphur)n-carbon bond in polysulphide cross-linked rubber is 27kcal/mol. The invention exploits the weaker carbon-sulphur andsulphur-sulphur bonds by applying a three dimensional strain to asurface layer of the rubber particle that exceeds the breaking strain ofthe cross-links while not exceeding the breaking strain of thecarbon-carbon bonds along the back bone of the rubber molecule.

The selection of temperature and pressure of the supercritical fluidduring swelling are chosen such, that the solubility parameters δ of therubber and the supercritical fluid are matched.

The solubility parameter of a fluid is normally measured in terms of theHildebrand solubility parameter, which is defined as follows:

δ=[(ΔH−RT)/V _(m)]^(1/2)

-   where δ is the Hildebrand solubility parameter (MPa)^(1/2)-   ΔH is the heat of vaporisation (KJ mol⁻¹)-   R is the universal gas constant (KJ mol⁻¹ K⁻¹)-   T is temperature (Kelvin)-   V_(m) is the molar volume (dm³ mol⁻¹).

In general, substances having a similar value of solubility parametermix well. In the case of a fluid and a polymer having a similarsolubility parameter, the polymer will generally dissolve well in thefluid. However, in the case of a fluid and a polymer, where the polymerexhibits a substantial degree of cross-linking (such as vulcanisedrubber), the polymer does not dissolve in the fluid. Instead, the fluidpermeates the polymer causing swelling.

Thus, in the case of a supercritical fluid (e.g. CO₂) diffusing into avulcanised rubber, the selection of the process conditions controls thedegree to which the supercritical fluid permeates the rubber. From theabove equation, it can be seen that the solubility parameter of thesupercritical CO₂ may be adjusted by modifying the pressure or thetemperature of the fluid. Increasing the temperature will decrease thesolubility parameter of the supercritical CO₂. By contrast, increasingthe pressure will increase the solubility parameter. Therefore, byadjusting the temperature and pressure according to above equation, thesolubility parameter of the supercritical fluid and that of thevulcanised rubber can be matched to maximise the swelling of thevulcanised rubber. It is desirable to maximise the swelling of therubber because this means that the sulphur-containing cross links areextended before the external pressure is reduced, so that when thispressure drop takes place, the maximum internal pressure is applied tothe sulphur containing cross links causing the cross links to exceedtheir point of maximum extension, resulting in their rupture.

Starting from conditions under which the supercritical fluid exhibits alower solubility parameter than the vulcanised rubber, the level ofequilibrium swelling of the rubber will increase as the solubilityparameter of the supercritical fluid increases towards the solubilityparameter value of the rubber material. As the solubility parameter ofthe supercritical fluid approaches that of the vulcanised rubber, thedegree of swelling of the vulcanised rubber will level off beforebeginning to decrease as the solubility parameter of the supercriticalfluid increases beyond that of the vulcanised rubber.

Typical solubility parameters of rubbers are in the range 14-22MPa^(1/2) with polar vulcanised rubbers tending to have a value ofsolubility parameter at the higher end of this scale. For supercriticalCO₂ at a temperature of about 305K (just above the critical point) thiscorresponds to a pressure range between about 10 and 40 MPa. However, inpractice, the degree of swelling is observed to increase relativelyrapidly with pressure up to about 10 MPa.

The application of high pressures does increase the cost of the processand so in some embodiments, it is preferred that the initial pressure beless than the pressure required to obtain the maximum level ofequilibrium swell, for example the pressure level may be set at orslightly above the level at which the variation of swelling withincreased pressure begins to level off. The initial pressure levelselected will therefore depend on the vulcanised rubber but will usuallybe less than 40 MPa, such as less than 30 MPa, for example less than 20MPa or less than 15 MPa e.g. less than 12 MPa. The initial pressurelevel must be above the supercritical level of the fluid and so ispreferably greater than 8 MPa, for example greater than 9 MPa, such asgreater than 10 MPa.

As well as the pressure, it is necessary to control the temperature ofthe supercritical fluid. Since the solubility parameter decreases withincreasing temperature, it is generally preferred that the initialtemperature be maintained at as low a level as will allow the fluid toremain supercritical, in order to reduce the pressure required toachieve a given solubility parameter. However, in some embodiments,particularly those in which it is desired that the fluid remainsupercritical following the initial pressure reduction step, the initialtemperature may be higher.

In the case of CO₂, the critical temperature is 31.1° C. and so,depending on the embodiment, the initial temperature of the CO₂ ispreferably maintained at a temperature greater than 32° C., for examplegreater than 40° C., such as greater than 50° C., or greater than 60° C.The initial temperature of the CO₂ will also usually be less than 140°C., for example less than 120° C., such as less than 100° C., or lessthan 80° C.

As mentioned above, at a given temperature, the degree of swelling ofthe vulcanised rubber increases relatively quickly with pressure up to agiven pressure value and then more slowly as pressure increases beyondthis value. This has the advantageous effect that at relatively highpressures, the degree of swell obtainable following an initial pressuredrop is only slightly less than that obtainable at the initial pressure.

This enables the use of a multistage pressure reduction in which thepressure is reduced as previously specified and then the rubber allowedto reach equilibrium swell before the pressure is reduced again. Theswelling process following the initial pressure reduction takes placemuch more quickly where the fluid remains in a supercritical state thanwhen the fluid becomes sub critical on the pressure reduction. Thisallows for an increased yield of de-vulcanised rubber in a shortertimescale than would be the case if the fluid needed to bere-pressurised in order to become supercritical and re-permeate therubber.

A more detailed description of the invention will now be given withreference to the accompanying drawings in which:

FIG. 1 Shows a representation of relaxed cross-linked rubber moleculesin normal ambient conditions.

FIG. 2 Shows a representation of cross-linked rubber molecules in aswollen state in supercritical fluid at an appropriate temperature andpressure to maintain a supercritical state.

FIG. 3 Shows a representation of de-vulcanised rubber molecules andbroken cross-links after a rapid decompression of the supercriticalfluid causes a three dimensional separation of the molecules beyond theextendable limit of the cross-links, thereby causing the cross-links tofracture.

FIG. 4 Shows a representation of the processing equipment in which tocarry out the functional operations of the invention.

Central to the invention is that supercritical fluids display uniquephysiochemical properties: they are of low viscosity, high diffusivityand have high thermal conductivity. Under the correct conditions oftemperature and pressure the solubility parameter of supercriticalfluids can be controlled to closely match the solubility parameter ofvulcanised rubber, making the supercritical fluid an excellent solventfor vulcanised rubber, allowing easy impregnation into the rubbers threedimensional molecular network, causing it to swell. Among severalsupercritical fluids, CO₂ is the most advantageous for this invention,having an easily obtainable critical point and being chemicallyinactive, non-toxic, non-flammable and inexpensive.

FIG. 1 shows a representation of vulcanised rubber in a relaxed stateconsisting of long chained molecules (1) held together by some chainentanglement (2), but predominantly by the presence of chemicalcross-links (3) between adjacent molecules. It is the combination ofhighly extendable long chained molecules and chemical cross-links thatgives rubber its unique elastic properties. Without the influence ofchemical cross-links, rubbers would be predominantly plastic by natureand would be easily dissolved by suitable solvents to form free flowingsolutions. Within the present invention, the rubber would preferably besourced from vulcanised process waste generated from rubber processingindustry or from end of life vulcanised rubber products. Also within thepresent invention the vulcanised rubber would preferably undergo aprocess of grinding prior to the process described herein as theinvention to form a particulate of the vulcanised rubber. Theparticulate size and shape is not critical to the outcome of theinvention process, although its efficiency will be affected. Smallerparticles offer an improved surface area to volume ratio for theeffectiveness and efficiency of the invention. Typically, the particlesize of the vulcanised rubber particulate might be from 10 mm diameterreducing to 0.038 mm (ASTM mesh No. 400), but would preferably be withinthe range of 1.00 mm (18 mesh) to 0.15 mm (100 mesh), and morepreferably be within the range of 0.425 mm (40 mesh) to 0.18 mm (80mesh) which offers a balance of useful surface area to volume ration andreasonable grinding cost.

The vulcanised rubber, preferably in particulate form, as described, isplaced into a temperature controllable pressure vessel such as anautoclave. Carbon dioxide in gas or liquid form, but preferably inliquid form is pumped into the pressure vessel at or above its criticalpressure of 7.4 Mpa. The temperature of the interior of the pressurevessel is then increased to, or above the supercritical temperature ofcarbon dioxide, being equal to or greater than 31.1° C. so that thecarbon dioxide becomes a supercritical fluid and fills the autoclavechamber. At supercritical conditions the density of carbon dioxide as asupercritical fluid is 0.469 g/cm³. The density of the supercriticalfluid, its diffusivity into polymeric materials and its solubilityparameter is, however, highly dependent upon changes to both temperatureand pressure above the critical points. This has been described by M.Kojima, M. Tosaka, E. Funami, K. Nitta, M. Ohshima, S. Kohjiya, in“Phase Behaviour of Crosslinked Polyisoprene Rubber and SupercriticalCarbon Dioxide” allowing someone trained in the art to select an optimumset of conditions so as to match the solubility parameter of thevulcanised rubber to be processed, thereby enabling conditions for rapidadvancement to equilibrium swell. FIG. 2 portrays the vulcanised rubberat the surface of a rubber particle under conditions close toequilibrium swell in a supercritical fluid (4). The rubber molecules (1)are separated by the diffusion of the supercritical fluid (5), butinhibited from further separation by the three dimensional network ofchemical cross-links (3). As the restraining influence on the rubbermolecules, the cross-links will be under their maximum strain when therubber is at equilibrium swell.

A main embodiment of this invention permits the near equilibrium swellof the outer surface of the vulcanised rubber particulate only. As thesupercritical fluid will migrate through the outer surface towards thecentre of each vulcanised rubber particle, the outer surface of eachvulcanised rubber particle will reach a state of near equilibrium swellbefore such a state is achieved at the centre of the rubber particle.Subsequent processing described herein within the scope of the inventionwill thereby achieve de-vulcanisation of the outer crust of each rubberparticle, allowing the retention of the rubber's original physicalproperties within the core of each rubber particle. Later inclusion ofthe treated particulate rubber as an additive to a new rubber mix willpermit chemical cross-linking between the de-vulcanised outer surface ofthe rubber and the new rubber matrix.

The surface depth of de-vulcanisation will vary depending on theparticle geometry but will typically involve significantde-vulcanisation (i.e. breakage of greater than 10%, for example greaterthan 30% or greater than 50% of sulphur containing bonds in a region) toa depth of less than 20% of the largest particle diameter, for exampleless than 10%, such as less than 5%.

A further embodiment of this invention exploits the excellent solventand dissipation properties of supercritical fluids to act as a carrierfor active additives such as chemical de-vulcanising agents. Ade-vulcanizing agent is preferably selected from phenyl-hydrazine-ironchloride, triphenyl phosphine, thiols and disulphides. The use of suchadditives assists with the processes of de-vulcanisation throughchemical cleavage of a proportion of sulphur cross-links prior tophysical cleavage through the preferred embodiment of this invention

A further advantage of this invention again is based on the excellentsolvent and dissipation properties of supercritical fluids.Supercritical fluids will therefore extract low molecular weightmaterials commonly found within typical vulcanised rubber compounds,examples of which being monomers, oils, waxes and soaps which,individually or in combination migrate to the surface of vulcanisedrubber and act to form a surface contaminant that reduces or destroysboth physical and chemical bonds at the rubber/adhesive interface. Suchextraction of low molecular weight contaminants found within vulcanisedrubber will in itself serve to improve the compatibility of vulcanisedrubber into a new rubber mix matrix, by improving interface wettability,leading to a greater intimacy of surface contact.

Once a swell of the surface is achieved by choosing a time of swellingthat is not more than 50% of the time required for full volumeequilibrium swelling, the internal pressure of the pressure vessel israpidly dropped. The supercritical fluid around the vulcanised rubberwill revert to a gaseous state (5), causing a sudden pressure differencebetween the autoclave chamber pressure and the internal pressure withinthe surface of the vulcanised rubber particle, due to supercriticalfluid remaining partially trapped between the rubber molecules. In anattempt to achieve a pressure equilibrium, the supercritical fluidbetween the rubber molecules will rapidly expand (6), forcing separationof the rubber molecules (1) beyond the restraining capability of thecross-links, causing the cross-links to fracture (7).

Deeb, Victor M and Rouse, Michael W in U.S. Pat. No. 6,680,110 describea method for size reduction of particulate materials, including rubbers,by allowing a rapid pressure drop to atmospheric pressure after swellingsaid materials in a supercritical fluid to equilibrium conditions. FIG.3 shows that under these conditions the amount of extractable materialsis hardly more than the amount of extractable materials present in theoriginal material.

A sudden reduction of the pressure that is applied after equilibriumswelling,causes an explosive decompression with breakage of the longchained rubber molecules, thus tearing the rubber particle apart, asused to good effect by Deeb et. Al.

A reduction of the pressure after swelling during a period which is notmore than 50% of the time required for full volume equilibrium swellingof the particle causes de-vulcanization at the surface of the vulcanizedparticle only. Preferably set period is not more than 40%, morepreferably not more than 30% of the time required for full volumeequilibrium swelling.

Preferably the rapid decompression of the partly swollen vulcanisedrubber particle is controlled in order to maximise the breaking of thesulphur-containing linkages whilst avoiding macroscopic structuraldamage to the rubber (i.e. as could result from explosivedecompression), the absolute initial and final pressure values are lessimportant than the absolute pressure difference of the decompressionstep.

The structure of the vulcanised rubber may be considered to be a systemof entangled rubber molecules, in which the molecules are cross-linkedby sulphur-containing groups, effectively forming free volume into whichthe supercritical fluid may penetrate.

Following a reduction in external pressure, the internal pressure of avulcanised rubber particle can be released in a number of ways:

-   (i) diffusion of the fluid from the rubber particle;-   (ii) reorganisation of the rubber polymer chains to allow the fluid    to leave the free volume more quickly;-   (iii) breaking of the sulphur-containing bonds to allow the free    volume to expand thereby reducing the internal pressure of the    particle;-   (iv) macroscopic damage to the rubber structure forming pores that    allow the pressure to escape.

As seen previously, the bond energies of the sulphur containing crosslinks are smaller than those of the polymeric carbon-carbon bonds (27-60Kcal mol⁻¹ vs 93 Kcal mol⁻¹). Therefore, the degree to which thedifferent bonds are broken depends in part on the initial pressuredifference between the inside of the vulcanised rubber particle and theexternal system.

During the initial loading of the vulcanised rubber with thesupercritical fluid, equilibrium swell is attained when the expansion ofthe free volume within the rubber is equally constrained by thesulphur-containing cross links.

Following the depressurisation step, if the pressure difference issufficiently low, the strength of the cross links will be sufficient toretain the internal connectivity of the rubber and the fluid willdiffuse out of the particle, with perhaps some rearrangement of thepolymer particles to allow more rapid equilibration.

It seems that a minimum pressure drop of at least 5% is required tobreak the weakest bonds constraining the expansion of the free volume inthe outer layer (polysulphide bonds). If the pressure drop is less thanthis minimum value, the fluid will simply diffuse out of the rubberparticle without changing its structure.

In order to increase the proportion of sulphur-containing cross linksbroken, the pressure drop may be increased. As the size of the pressuredrop increases further, the resulting pressure difference will becomesufficient to begin to break the stronger sulphur-containing cross linkssuch as the C—S—S—C disulphide bond and then the C—S—C monosulphidebond. At this stage, the yield will still be increasing as the pressuredrop increases. However, depending on the level of surface swelling thepressure drop will be sufficiently large that, in order to equilibratethe internal and external pressure, the de-vulcanised rubber will beginto suffer macroscopic damage. At this stage, the yield will begin todecrease due to the fluid creating microscopic pores in the particlefrom which it can escape, thereby relieving the pressure on the crosslinks. As mentioned above the pressure drop takes place on a timescalethat is rapid compared to the rate of diffusion of the supercriticalfluid from the particle in order to minimise internal pressure loss bydiffusion.

The rate of pressure drop at its greatest rate would be virtuallyinstantaneous and governed by the design of the process, but mayadvantageously be regulated and controlled as a function of time.

As mentioned above, following a depressurisation step,sulphur-containing bonds in the region of free volume closest to thesurface of the rubber particle are broken allowing this region of freevolume to expand until the pressure within the region reduces to a levelthat is no longer sufficient to break the weakest polysulphide bondsconstraining further expansion of the free volume in this region. It ispossible to increase the number of bonds broken by increasing the sizeof the pressure drop. However, this will eventually result in a decreasein yield due to macroscopic damage to the structure of the rubber takingplace.

However, by controlling the rate at which the external pressure isreduced, it is possible to maintain a pressure difference between theexternal system and a free volume in the particle that is sufficientlyhigh to allow further sulphur-containing bonds to be broken once theinitial expansion of the outer region of free volume has taken placewhilst minimising any damage to the structure of the rubber.

Since the severity of the decompression experienced by the particle isreduced by reducing the rate at which the pressure drops, a largerpressure difference may typically be used for embodiments in whichreduction of pressure from its initial value to its final value takesplace over a longer timescale.

The pressure drop is typically greater than 2 bar, for example greaterthan 0.5 MPa such as greater than 1 MPa or greater than 1.5 MPa. Thepressure drop would also typically be less than 30 MPa, such as lessthan 20 MPA or less than 10 MPa. The pressure drop may also be less than5 MPa, such as less than 3 MPa, for example less than 2.5 MPa.

In embodiments in which the pressure drop takes place over a shorttimescale, for example less than 5 seconds, the pressure drop isgenerally selected to be towards the lower end of this range. Ingeneral, this pressure drop is less than 10 MPa, such as less than 5MPa, for example less than 3 MPa, or less than 2.5 MPa. The pressuredrop is also generally greater than 0.2 MPa, for example greater than0.5 MPa, such as greater than 1.0 MPa, or greater than 1.5 MPa.

In general the pressure drop would take place over a timescale ofgreater than 0.5 seconds, for example greater than 1 second, such asgreater than 2 seconds. The pressure drop would also typically takeplace over a timescale of less than 60 seconds, for example less than 30seconds, such as less than 15 seconds or less than 5 seconds.

In embodiments in which the pressure drop takes place over a timescaleof less than 60 seconds, the pressure drop may take place over atimescale of greater than 5 seconds, for example greater than 15seconds, such as greater than 30 seconds.

In embodiments in which the pressure drop takes place over a timescaleof less than 30 seconds, the pressure drop may take place over atimescale of greater than 5 seconds, for example greater than 15seconds.

In embodiments in which the pressure drop takes place over a timescaleof less than 15 seconds, the pressure drop may take place over atimescale of greater than 5 seconds.

In embodiments in which the pressure drop takes place over a timescaleof less than 5 seconds (for example less than 4 seconds or less than 2seconds), the pressure drop may take place over a timescale of greaterthan 0.5 seconds, for example greater than 1 second.

In most embodiments, the time taken for the pressure to drop from itsinitial value to its final value would be less than 1/10^(th) of thetime required for the pressure to equilibrate by diffusion, for exampleless than 1/50^(th), such as less than 1/100^(th) or less than1/500^(th).

It is therefore possible to optimise the yields by selecting the sizeand rate of the pressure drop. However, at the highest yield, it islikely that some damage is being done to the structure of the rubber.

In a further embodiment of the invention, the yield of de-vulcanisedrubber may be improved by employing a multistage process in which morethan one pressure drop at the preferred rate is employed. Thismultistage process comprises applying a controlled pressure drop andrate as in the single stage process and then applying a furthercontrolled pressure drop and rate. This process may be repeated asrequired.

In this way, further sulphur-containing cross linkages may be brokenwhilst minimising damage to the structure of the rubber, therebyincreasing the yield of de-vulcanised rubber.

It is preferred that following each pressure drop, surface swell isattained before the next pressure drop is applied. This reduces thepressure drop required to break the sulphur-containing bonds because itminimises the expansion of the free volume required to put strain on thecross linkages. For this reason, it is further preferred that theoverall pressure range is selected so that the supercritical fluidsolubility parameter remains as close as possible to that as thevulcanised rubber. This allows the swelling of the rubber at the surfaceof the particle to be maximised during each stage. If the solubilityparameter of the rubber is too high to be conveniently achieved withoutthe use of excessively high pressure, the initial pressure of themultistage process may be selected so that following the initialpressure drop, the solubility parameter of the supercritical fluidremains within 2 MPa^(1/2) of the initial solubility parameter,preferably within 1 MPa^(1/2), more preferably within 0.5 MPa^(1/2).

It is also preferred that the fluid remains supercritical during themultistage process to allow more rapid equilibrations between stages.

The larger the number of pressure drop steps the larger the yield ofde-vulcanised rubber. However the successive additional yield decreaseswith each step. In addition the process time increases as the number ofdepressurisation steps increases. The typical number of pressure dropsin the multistage process will therefore usually be fewer than 10, forexample fewer than 5 such as 2-4.

The size and rate of each pressure drop is selected to minimise damageto the rubber, whilst being large enough to cause breakage ofsulphur-containing cross linkages.

In embodiments where the pressure drop takes place relatively quickly(for example over a timescale of less than 5 seconds) the pressure dropwill typically be greater than 0.2 MPa, for example greater than 0.5MPa, such as greater than 1.0 MPa. The pressure drop will also usuallybe less then 3.0 MPa, for example less than 2.5 MPa, such as less then2.0 MPa or less than 1.5 MPa.

In embodiments where the pressure drop takes place relatively slowly,(for example over a timescale of 5 to 60 seconds) the pressure drop willtypically be greater than 1.0 MPa, for example greater than 1.5 MPa,such as greater than 2.0 MPa or greater than 2.5 MPa. The pressure dropwill also usually be less than 30 MPa, such as less than 20 MPa or lessthan 10 MPa. The pressure drop may also be less than 5 MPa.

In addition, as described previously, the initial temperature of thefluid should be selected so that, in embodiments where it is requiredthat the fluid remains supercritical, any temperature reductionassociated with the pressure drop should be small enough to enable thefluid to remain supercritical.

Following the de-vulcanisation process of the invention, the product maybe employed in general rubber applications or alternatively, furtherprocessed, for example by solvent extraction, in order to separate thede-vulcanised material from the remaining vulcanised material in therubber material.

Solvent extraction may be carried out using any solvent whichselectively removes the de-vulcanised rubber from the rubber material.The solvent preferably exhibits a solubility parameter in the range 15to 18 MPa^(1/2) for non-polar rubbers and may for example be toluene,and a range of 18 to 24 MPa^(1/2) for polar polymers and may for examplebe methyl ethyl ketone.

The de-vulcanised rubber of the invention comprises a number ofadvantageous properties.

For example, higher addition levels of the de-vulcanized rubber as anadditive to a new rubber compound can be made, compared tonon-de-vulcanized rubber, while retaining superior processing propertiesand vulcanized physical properties.

The process of the invention increases the level of solvent extractablematerial within the de-vulcanized rubber when compared withnon-de-vulcanized rubber. The solvent extractable material, havingvisco-elasticity and tack contributes to easier incorporation of thede-vulcanized rubber into a new rubber mix, thereby permitting higheraddition levels to a new rubber mix when compared with non-de-vulcanizedrubber.

In addition, the de-vulcanized rubber exhibits increased chemicalactivity with respect to free radicals, when compared to vulcanizedrubber, thereby permitting chemical cross linking within a new rubbermatrix. This gives enhanced vulcanized physical properties to thecompound when compared to the use of equivalent addition levels ofvulcanized rubber.

An additional advantage of the present invention can be obtained, when ade-vulcanized saturated rubber is mixed in an unsaturated rubber likeNR, SRB, BR, or a mixture of these.

A saturated rubber in this description is understood to be a rubber witha saturated backbone. Examples of saturated rubbers are ethyleneα-olefin copolymers comprising one or more non-conjugated polyenes.Examples of α-olefins are for instance α-olefins with 3-10 carbon atoms,like propylene, butylene, hexene, octene etc. Preferably, propylene isused (EPDM). Examples of non-conjugated polyene are5-ethyliden-2-norbornene, 5-vinyl-2-norbornene, dicyclopentadiene, 1,4hexadiene or mixtures thereof.

The poor UV and ozone resistance is a well known problem of unsaturatedrubbers. It has been described by e.g. Kannika Sahakaro, et. al inJournal of Applied Polymer Science, Vol. 103, 2555-2563 (2007). KannikaSahakaro, et describe that EPDM incorporated into blends of naturalrubber/butadiene rubber (NR/BR) improves ozone resistance. The inferiormechanical properties of NR/BR/EPDM blends generally obtained byconventional straight mixing are overcome by utilizing a reactiveprocessing technique.

An alternative for this cumbrous reactive processing technique is givenby the present invention by using the de-vulcanized saturated rubber asa component of a new rubber mix, further comprising a base polymer, anactivator system, accelerator system and cross-linking agent andoptionally structural and non-structural fillers, processing oils,process aids, and or a protective system.

Preferably the de-vulcanized saturated rubber in such a new rubber mixis EPDM, due to its excellent UV and ozone resistance. EPDM isparticularly advantageous in a new rubber mix, wherein the base polymeris SBR, BR, NR or a mixture of these.

FIG. 4 shows the basic components of the process necessary to carry outdecompression de-vulcanisation as described within the process of thepresent invention. The pressure vessel (8) would preferably be capableof operating between temperatures of 20° C. to 175° C. while maintaininga temperature control of +/−1° C., and preferably capable ofwithstanding internal operating pressures from 0 MPa to 20 MPa.Vulcanised rubber, preferably in particulate form, and preferably beingof a particulate size of 0.425 mm to 0.18 mm across its largestdimension is loaded into, and sealed within the pressure vessel. Thevolume of vulcanised rubber loaded into the pressure vessel must besmall enough to allow the vulcanised rubber to reach equilibrium swell,and to allow further expansion due to subsequent rapid decompression.The level of swell achievable by a vulcanised rubber will varyconsiderably depending upon the components used within the rubberformulation. The type of base polymer, different fillers and loadings,and the type and density of cross-links will all affect the swellbehaviour of vulcanised rubber in a solvent. Tests will therefore benecessary to establish the level of swell achievable by any given type,batch, grade or quality of vulcanised rubber. It is normal however thatthe initial volume of vulcanised rubber within one processing batch willbe less than 20% of the internal volume of the pressure vessel.

The pressure vessel (8) would be served by an inlet pipe (9) andnon-return valve (10) suited to the delivery of gasses and fluids,through which a gas or a liquid, being preferably a liquid and further,being preferably liquid carbon dioxide, is pumped from a storage vessel(11) by a suitable high pressure pump (12) into the pressure vessel upto or beyond the supercritical pressure of the gas or liquid which, forcarbon dioxide is a pressure of 7.4 MPa. Once the supercritical pressureis achieved, heaters within the pressure vessel increase the temperatureof the liquid gas up to or beyond the supercritical temperature whichfor carbon dioxide is 31.1° C., causing the liquid gas to change into asupercritical fluid. The supercritical fluid will completely fill thepressure vessel and will come into intimate contact with the vulcanisedrubber. The pressure within the pressure vessel will then be increasedas necessary by an operator skilled in the art, to achieve the optimumsolubility performance for the vulcanised rubber being processed.Pressure and temperature is maintained to preserve optimum conditionsfor a period sufficient to achieve equilibrium swell.

Once the vulcanised rubber has reached a state of equilibrium swell, aquick release valve (13) is opened permitting the rapid escape of thepressurised supercritical fluid, and a consequent sudden drop in theinternal pressure of the pressure vessel. Control of the pressure dropis achieved by not venting to atmosphere, but rather by venting to areceiver (14) of adjustable internal volume (15), having the effect ofsuddenly increasing the internal volume of the pressure vessel. It is anaspect of the present invention that the receiver has an adjustableinternal volume, determined by the use of different sized receivers, orpreferably by the use of an adjustable sealed internal plate. The volumeof the receiver dictates the level of pressure drop within the pressurevessel, and therefore the extent of the three dimensional expansion ofthe processed rubber. The larger is the internal volume of the receiver,the greater will be the pressure drop and the extent of the threedimensional expansion experienced by the processed rubber.

A further aspect of the present invention is that once the rapidpressure drop has been achieved, a further slow pressure drop iscontrolled by opening a bleed outlet valve (16) on the receiver,allowing a controlled pressure drop of gas to atmosphere, or preferablythrough a cooler (17) to convert the gas back to a liquid for pumpingback (18) to the storage vessel (11), permitting closed loop recyclingof the processing gas. The rate of the slow pressure drop must becontrolled to permit the release of supercritical fluid from within theprocessed rubber without causing physical damage to the processedrubber, and must preferably match the rate of diffusion of thesupercritical fluid from the processed rubber as it converts to gas.

When the pressure within the pressure vessel is reduced to a pressureequal to atmospheric pressure, the pressure vessel may be opened, andthe processed rubber, now in a predominantly de-vulcanised state may beremoved.

De-vulcanised rubber resulting from the process described within theembodiments of this invention can be processed by someone skilled in theart of using conventional rubber mixing, forming and curingtechnologies, as commonly found within the rubber processing industry,including formulating, mixing, milling, calendaring, extrusion,compression moulding, transfer moulding, injection moulding, fabricationand pressureless curing techniques. Reuse of the fully de-vulcanisedrubber may be achieved by mixing the de-vulcanised rubber with a curesystem consisting of activators, accelerators and a cross-linking agentsuch as sulphur, selected by someone skilled in the art to achievedesired processing and curing characteristics. Other ingredients such asfillers, processing oils, plasticizers, and miscellaneous protectivesystems may also be added as required to achieve desired physical andprocessing properties. A further use of the de-vulcanised rubber is asan additive into a new rubber mix, acting as a partial replacement forvirgin compounding ingredients including polymer, reinforcing filler andnon-reinforcing filler. Use of the de-vulcanised rubber may be achievedin this way by mixing the de-vulcanised rubber as a proportion of thewhole mix with conventional ingredients such as polymer, reinforcing andnon-reinforcing fillers, process oils, process aids, miscellaneousprotective systems and a cure system consisting of activators,accelerators and a cross-linking agent such as sulphur, selected bysomeone skilled in the art to achieve desired processing and curingcharacteristics.

Preparation of the compound can be carried out in a conventional mixer,e.g. an internal mixer having tangential rotors or intermeshing mixingelements. Other kinds of internal mixer or other mixer may be used; itis a feature of the present technology that special equipment isgenerally not required. Equally, the sequence of addition of compoundingredients may be generally conventional.

Fill factor and addition times can be established in line withconventional skill to suit the machine in which the compound is mixed.Dump temperature should be controlled to maintain scorch safety, e.g. tobelow 110° C. The temperature in the mix during mixing (arisingprimarily from shear forces) is typically between 100° C. and 150° C.Mixing time is typically between 1 and 8 minutes.

Once mixed, the compound containing the dispersed rubber crumb forms acoherent processable batch which can be discharged from the mixer onto asuitable processing apparatus such as a two-roll open mill. In line withconventional practice, further dispersion of the components can beachieved by cutting and blending the mixed batch on the two roll mill.

From a two roll mill, the mixed batch can be passed as a continuouscoherent sheet which is able to support its own weight. This may then beprocessed in a conventional way, e.g. being passed through an anti-tackdip and allowed to cool before removal from the process and subsequentforming e.g. by compression moulding. Typically this may involveprocessing into a sheet of predetermined thickness and width using acalendar. This gives the necessary accurate dimensions for subsequentmoulding in a compression press, either by continuous feed of thecalendared sheet or by the use of moulding blanks cut from thecalendared sheet. Typical moulding is carried out at a temperaturebetween 130° C. and 180° C. using a closing force sufficient to fullyform the desired moulded product.

1. A method for de-vulcanizing a vulcanized rubber comprising: swellingsaid material in a supercritical fluid maintained at a supercriticalpressure and temperature, whereby said supercritical fluid causesswelling of the said vulcanized rubber material, and a subsequent rapidreduction of the supercritical fluid pressure characterized in that thetime of swelling is not more than 50% of the time required for fullvolume equilibrium swelling of the rubber in said supercritical fluid.2. A method according to claim 1, further comprising adding an additiveto the supercritical fluid before allowing said vulcanized rubber toswell.
 3. A method according to claim 1, wherein the rubber is EPDM. 4.A method according to claim 13, wherein the base polymer is SBR, BR, NRor a mixture of these.
 5. A method according to claim 1, wherein thepressure reduction in said rapid reduction of supercritical fluidpressure is more than 5%.